When to sow cucumbers

Cucumbers hate the cold. Frost kills them outright, and even a run of chilly nights below about 54°F stalls their growth for weeks. So the calendar is set by the last frost, not by how early you can get seed to sprout.

For cucumbers that will grow outdoors, sow indoors from mid-April to early May. The seedlings are ready to go out three to four weeks later, which lands after the Eisheiligen in mid-May in Germany, Austria and Switzerland, and in late May or early June in most of the UK. Sowing earlier only gives you leggy plants waiting on a windowsill.

For an unheated greenhouse or polytunnel, sow in April and plant in from mid-May. In a heated greenhouse you can start as early as late February or March.

You can also sow straight into the bed once the soil has warmed, from mid-May in mild areas to late May or early June elsewhere. Put two or three seeds at each spot and keep the strongest seedling.

Sow one or two seeds in a 4–4 in pot of seed or potting compost, 0.4–1.2 in deep and on their side. Keep them warm: germination is quickest at 68–82°F and usually takes one to two weeks, often less. Once they are up, grow them on a little cooler, around 68°F, in good light so they stay stocky.

Before planting out, harden the seedlings off for a week or so by putting them outside during the day. Plant them once they have a few true leaves, without burying the stem deeper than it sat in the pot, and water them in with water that isn't ice-cold.

Pick the sunniest, most sheltered spot you have and dig in plenty of compost or well-rotted manure first. Cucumbers are heavy feeders and need soil that stays moist but drains. Aim for a pH of about 6 to 7.

Training them up a trellis, netting or strings saves space, keeps the fruit clean and straight, and lets the leaves dry faster, which helps against mildew. Plants on a trellis need 12–16 in between them. Left to sprawl, outdoor cucumbers need rows 39–55 in apart. In a 120 × 80 cm raised bed, three or four plants on a trellis along one long side is plenty.

Cucumbers are mostly water, and they want an even supply. Keep the soil evenly moist and water at the base in the morning, so the leaves dry off during the day. Plants in pots or grow bags may need water every day in hot weather. A mulch of grass clippings or straw keeps the soil from drying out.

Once the first fruits are swelling, feed every one to three weeks. A general liquid feed works, or a high-potash tomato feed once flowering starts; diluted nettle tea is the organic route.

Greenhouse cucumbers grow tall fast. Tie the main stem to a string or cane and pinch out its tip when it reaches the roof. Pinch side shoots two leaves beyond a female flower, which is the one with a tiny cucumber behind it.

In a greenhouse, keep daytime temperatures around 64–77°F and the air humid by damping down the floor on hot days. Dry, hot air is what red spider mites like.

The first cucumbers are usually ready about two to three months after sowing: from July outdoors, earlier under glass, and on until the first cold nights of September. Once plants are cropping, pick two or three times a week. Fruit left to swell and yellow tells the plant it's done and it stops setting new ones.

Pick pickling cucumbers small, at about 2–6 in. Salad cucumbers are ready at 6–10 in, or when the sides have grown parallel and the skin is still firm and glossy. Yellow, bulging or soft fruits are past it. Cut them with a knife or secateurs rather than pulling, so you don't tear the stem.

Cucumbers don't like cold. In the fridge they soon turn soft and watery. Keep them cool, ideally around 50–55°F, and eat them within one to two weeks.

Outdoor or ridge cucumbers are the tough ones. Their fruit is shorter, often bumpy or spiny, and the plants cope with a cooler summer. They carry male and female flowers, so they need bees to set fruit. Leave the male flowers on. Pickling cucumbers (gherkins) belong here too; they are picked small.

Greenhouse cucumbers are the long, smooth salad cucumbers from the shop. Most modern varieties are all-female and parthenocarpic: they set fruit without pollination. They need warmth and shelter to do well, so outdoors they only really succeed in a warm summer.

If an all-female variety does throw the odd male flower, pinch it off. Pollinated fruit on these varieties can turn bitter and misshapen.

Between the two sit mini or snack cucumbers. Several are parthenocarpic yet robust enough for a warm, sheltered bed outdoors, which makes them a good choice for a raised bed.

Powdery mildew coats the upper side of the leaves in a white, floury film. Downy mildew shows as yellow, angular patches between the leaf veins with a greyish fuzz underneath, and it spreads fast in wet weather. Against both: water at the base in the morning, give plants air, and choose varieties sold as tolerant or resistant.

Cucumber mosaic virus causes yellow mottled, crinkled leaves, stunted plants and misshapen fruit. Aphids spread it. There is no cure, so pull and bin affected plants and keep the bed free of weeds that can carry the virus.

Red spider mites thrive in hot, dry air, especially under glass, and turn leaves pale and speckled. Slugs go for young plants, so protect them for the first few weeks.

Bitter fruit is a sign of stress: heat, cold, drought or erratic watering. The bitterness sits mostly at the stalk end and in the skin, so cutting off the end and peeling helps. Plenty of male flowers and no fruit early in the season is normal; the male flowers come first, and the female ones follow.

Cucumbers belong to the cucurbit family, with courgettes, pumpkins, squashes and melons. Leave three to four years before any of them grows in the same spot again.

Mixed-planting charts often put cucumbers next to dill, dwarf beans, lettuce, kohlrabi or radishes. We found no solid evidence for these pairings; they are tradition. Low crops at the foot of a cucumber trellis make good use of the space, as long as they don't compete for water.

What does have a reason: don't follow cucumbers with other cucurbits, which share mildews, viruses and soil problems.